1. Field of the Invention
The present invention relates to an electrolytic solution and, more specifically, to a non-cyanogen type electrolytic solution for plating gold, containing a gold salt as a supply source of gold and a non-cyanogen type compound.
2. Description of the Related Art
A plated gold film is excellent in electric characteristic, corrosiveness, soldering ability or others. Accordingly, when a circuit board used in a semiconductor device or the like is produced, copper patterns formed on a surface of the circuit board are subjected to electrolytic gold plating.
This electrolytic gold plating is usually carried out in an electrolytic plating solution containing a cyanogen compound.
In this regard, if it is desired that only a predetermined part of the patterns formed on the surface of the circuit board is subjected to gold plating, the circuit board, the part which not to be plated with gold being covered with resist, is dipped into a gold-plating electrolytic solution.
However, when an electrolytic solution added with a cyanogen compound is used as a bath for gold plating, cyanogen ions corrode the resist to separate the latter from the surface of the circuit board. Thereby, the gold-plating electrolytic solution may enter a gap between the circuit board and the resist to form a gold film on a part of the circuit board which is not to be plated with gold.
Accordingly, when gold is plated to a predetermined portion of micro-patterns formed on the circuit board, gold is also plated to other portions not to be plated due to the entry of the gold-plating solution into the gap between the surface of the circuit board and the resist, which may cause a short-circuit between the micro-patterns.
To solve this problem, there has been proposed a non-cyanogen type electrolytic solution, containing a gold salt, as a supply source of gold, and a non-cyanogen type acetylcysteine as a complexing agent (see Japanese Unexamined Patent Publication No. 10-317183; pages 4 to 5).
According to the non-cyanogen type gold-plating electrolytic solution disclosed in the above-mentioned patent publication, it is possible to plate, with gold, only a predetermined portion of micro-patterns formed in a circuit board, the solution being less toxic and easily treatable as well as not being corrosive, due to cyanogen ions, to a resist coated on the circuit board because no cyanogen compound is added thereto. Thus, it is possible to plate gold on a predetermined portion of the micro-patterns formed on the circuit board.
However, it has been found that the gold-plating film obtained by using the non-cyanogen type gold-plating electrolytic solution disclosed in the above-mentioned patent publication is black in appearance and the gold-plating electrolytic solution bath lacks stability.